Leds driver

ABSTRACT

The power supply ( 20 ) for LEDs provides power to a LED light source ( 10 ) having a variable number of LEDs wired in series and/or in parallel. The power supply ( 20 ) uses current and voltage feedback to adjust power to the LEDs and provides protection to the LED light source ( 10 ). A feedback controller ( 27 ) compares sensed current and sensed voltage to a reference signal and generates a feedback signal, which is processed by a power factor corrector ( 124 ) to adjust the current flow through the transformer supplying current to the LEDs. A LED control switch ( 24 ) clamps a peak of the current to the LEDs to provide further protection to the LED light source ( 10 ). A short/open detection circuit ( 30 ) indicates any detection of a “LED outage” of the LED light source ( 10 ).

The technical field of this disclosure is power supplies, particularly,a power supply for LEDs.

Significant advances have been made in the technology of white lightemitting diodes (LEDs). White light LEDs are commercially availablewhich generate 10-15 lumens/watt. This is comparable to the performanceof incandescent bulbs. In addition, LEDs offer other advantages such aslonger operating life, shock/vibration resistance and design flexibilitybecause of their small size. As a result, white light LEDs are replacingtraditional incandescent sources for illumination applications such assignage, accenting, and pathway lighting. The white LEDs can be usedalone or in conjunction with colored LEDs for a particular effect.

The electrical characteristics of LEDs are such that small changes inthe voltage applied to the LED lamp will cause appreciable currentchanges. In addition, ambient temperature changes will also result inLED current changes by changing the forward drop across the LEDs.Furthermore, the lumen output of LEDs depends on the LED current. Theexisting electrical power supplies for LED light sources are notdesigned to precisely regulate the LED current to prevent luminousintensity variations due to input ac voltage variations and ambienttemperature. Operation of LED lamps at excessive forward current for along period can cause unacceptable luminous intensity variations andeven catastrophic failure. In addition, current electrical powersupplies do not minimize power consumption to maximize energy savings.

It would be desirable to have a power supply for LEDs that wouldovercome the above disadvantages.

One form of the present invention is a power supply for a LED lightsource that comprises a power converter and a LED control switch. Thepower converter operates to provide a regulated power including a LEDcurrent and a LED voltage. The LED control switch further operates tocontrol a flow of the LED current through the LED light source. The LEDcontrol switch further operates to clamp a peak of the LED currentduring an initial loading stage of the LED light source. This preventsdamage to the LED light source due to a field misapplication.

A second form of the present invention is a power supply for a LED lightsource further comprising a detection circuit operating to provide adetection signal indicative of an operating condition of the LED lightsource associated with the LED voltage. The detection signal has a firstlevel representative of a load condition of the LED light source. Thedetection signal has a second level representative of a short conditionor an open condition indicative of the LED light source.

A third form of the present invention is a power supply for a LED lightsource further comprising a LED current sensor or a LED voltage sensor.Each sensor includes a differential amplifier and means for adjusting again of the differential amplifier.

The foregoing forms as well as other forms, features and advantages ofthe present invention will become further apparent from the followingdetailed description of the presently preferred embodiments, read inconjunction with the accompanying drawings. The detailed description anddrawings are merely illustrative of the present invention rather thanlimiting, the scope of the present invention being defined by theappended claims and equivalents thereof.

FIG. 1 illustrates a block diagram of a power supply for an LED lightsource in accordance with the present invention;

FIG. 2 illustrates a schematic diagram of one embodiment of the FIG. 1power supply in accordance with the present invention;

FIG. 3 illustrates a timing diagram of one embodiment of a controlcircuit in accordance with the present invention;

FIG. 4 illustrates a schematic diagram of one embodiment of a short/opendetection circuit in accordance with the present invention; and

FIG. 5 illustrates a schematic diagram of one embodiment of adifferential amplification circuit in accordance with the presentinvention.

FIG. 1 illustrates a block diagram of a power supply 20 for powering anLED light source 10 including a variable number of LEDs wired in seriesand/or in parallel. A single-phase ac input 21 of power supply 20provides a voltage V_(AC) to an AC/DC converter 22 of power supply 20whereby AC/DC converter 22 converts voltage V_(AC) into a voltageV_(DC). AC/DC converter 22 provides voltage V_(DC) to a power converter23 of power supply 20 whereby power converter 23 generates a regulatedpower P_(REG) including a LED current and a LED voltage V_(LED).

Power converter 23 provides regulated power P_(REG) to LED light source10. In operation, LED control switch 24 controls a flow of the LEDcurrent through the LED light source 10. A LED current sensor 25 ofpower supply 20 provides a sensed current ISE indicative of a magnitudeof the LED current flowing through LED light source 10. A LED voltagesensor 26 of power supply 20 provides a sensed voltage V_(SE) indicativeof a magnitude of the LED voltage V_(LED) applied to LED light source10. Sensed current I_(SE) and sensed voltage V_(SE) are fed to afeedback controller 27 of power supply 20. A signal reference 28 ofpower supply 20 provides a reference signal REF to a feedback controller27, whereby feedback controller 27 provides a feedback signal FB topower converter 23 based on sensed current I_(SE), sensed voltage V_(SE)and reference signal REF.

LED control switch 24 further operates to clamp a peak of LED currentflowing through LED light source 10 to thereby protect the LED lightsource 10 from electrical damage. LED control switch 24 is particularlyuseful when LED light source 10 transitions from an open operating stateto a load operating state (i.e., an initial loading), such as, forexample, a connection of LED light source 10 to power supply 20subsequent to an energizing of power supply 20. An LED dimmer 29 ofpower supply 20 operates to control a desired dimming of LED lightsource 10 by providing a control signal CS to LED control switch 24.Control signal CS can be in one of many conventional forms, such as, forexample, a pulse width modulation signal (“PWM”).

A short/open detection circuit 30 provides a detection signal DS as anindication of a short condition or an open condition of LED light source10 based on the LED voltage V_(LED) applied to LED light source 10.

The configuration of each component 21-30 of power supply 20 is withoutlimit. Additionally, coupling among the components 21-30 of power supply20 can be achieved in numerous ways (e.g., electrically, optically,acoustically, and/or magnetically). The number of embodiments of powersupply 20 is therefore essentially limitless.

FIG. 2 illustrates a schematic diagram of one embodiment 120 of powersupply 20 (FIG. 1) for one embodiment 110 of LED light source 10(FIG. 1) made in accordance with the present invention. Power supply 120employs a flyback transformer with current feedback through a powerfactor corrector (“PFC”) IC to supply power to LED light source 110. Tothis end, power supply 120 includes an EMI filter 121, an AC/DCconverter (“AC/DC”) 122, a transformer 123, a power factor corrector124, a feedback controller 125, an optocoupler 126, a LED control switch127, a LED PWM dimmer 129, resistors R1-R7, capacitors C1-C5, diodesD1-D3, zener diodes Z1-Z3 and a MOSFET Q1 as illustrated in FIG.2.

Voltage is supplied to power supply 120 at V_(IN) to EMI filter 121. Thevoltage can be an ac input and is typically 50/60 Hertz at 120/230V_(RMS). EMI filter 121 blocks electromagnetic interference on theinput. AC/DC 122 can be a bridge rectifier and converts the ac output ofEMI filter 120 to dc. Transformer 123 includes a primary winding W1, W4and W5, and a plurality of secondary windings W2 and W3. The windingsW1/W2 constitute the flyback transformer to power the LED light source110. The flyback transformer is controlled by PFC 124, which is a powerfactor corrector integrated circuit, such as model L6561 manufactured byST Microelectronics, Inc.

The flyback transformer transfers power to LED light source 110 wherethe LED current and the LED voltage are controlled by feedback control.The forward converter operation of windings W1/W3 charge a capacitor C3and a reference current signal is generated between a series resistor R4and a zener Z2. The peak voltage across capacitor C3 depends on theW1/W3 turns ratio. The output dc voltage from flyback operation ofwindings W1/W2 cannot be used to generate the reference current signalsince the output dc voltage across LED light source 110 can have a widerange—from 2.6 Volts dc for one LED lamp to about 32 Volts dc for 8 LEDsin series. The forward converter operation of windings W1/W3 can be usedinstead. The forward converter operation of the W1/W5 windings can alsobe used to supply power to the integrated circuits, such as PFC 124.

A sensed LED current I_(SE) flows through resistor R1, which is inseries with the LED light source 110 via LED control switch 127. Avoltage representative of sensed LED current I_(SE) is applied to anon-inverting input of a comparator U1. A sensed LED voltage V_(SE) isgenerated by zener diode Z1. Sensed LED current I_(SE) and sensed LEDvoltage V_(SE) as well as a voltage reference V_(REF) are fed tofeedback controller 125,. whereby a voltage feedback V_(FB) fromfeedback controller 125 drives an optocoupler 126 via resistor R7. Ingenerating voltage feedback V_(FB), feedback controller 125 employs apair of comparators U1 and U2, resistors R8-R12, and a capacitor C6 asillustrated in FIG. 2.

Feedback controller 125 is necessary since optocouplers have a widerange of current transfer ratio (CTR). Feedback controller 125 maintainsan accurate voltage feedback V_(FB) to thereby avoid large errors in LEDcurrent flowing through LED light source 110. Optocoupler 126 isolatesthe dc circuit supplying the LED light source 110 from the ac circuitpower supply at EMI filter 120, the two circuits being on the oppositesides of the transformer 123.

The output of the optocoupler 126 is connected to PFC 124, whichsupplies a gate drive signal to MOSFET Q1. Control of MOSFET Q1 adjuststhe current flow through winding W1 of transformer 123 to match the LEDlight source 110 power demand. The internal 2.5 V reference signal andan internal compensation circuit of PFC 124 maintains the voltage dropacross a resistor R6 at 2.5V. Although this example uses MOSFET Q1 foradjusting the transformer current, alternate embodiments can use othertypes of transistors to adjust the current, such as an insulated gatebipolar transistor (“IGBT”) or a bipolar transistor. The input to PFC124 at Z_(CD) provides a reset signal powered from windings W2/W4.

Zener diode Z1 also provides overvoltage protection for LED light source110. Specifically, zener diode Z1 connects across the output connectionto the LED light source 110 and clamps the output voltage to a specifiedmaximum value. The nominal zener operating voltage is selected to bejust over the maximum specified output voltage. In case of an outputopen circuit, the flyback operation of windings W1/W2 of transformer 123would continue to build the output voltage. The increasing outputvoltage turns on the zener diode Z1 to thereby increase the amount offeedback to resistor R6 from feedback controller 125 via resistor R7 andoptocoupler 126. This limits the gate drive signal to MOSFET Q1,preventing the flyback converter from building the output voltage to theLED light source 110 beyond a specified maximum voltage. Similarly,zener diode Z3 connected from the reset winding W4 to resistor R6 willprevent output overvoltage due to a malfunction of feedback controller125. In alternate embodiments, either zener diode Z1 or zener diode Z3,or both zener diode Z1 and zener diode Z3 can be omitted depending onthe degree of control protection required for a particular application.

LED control switch 127 includes a switch SW1 in the form of a MOSFET anda switch SW2 in the form of a bipolar transistor. Switches SW1 and SW2can be in other conventional forms, such as, for example, an IGBT. Asillustrated, a drain of MOSFET switch SW1 is connected to LED lightsource 110. A gate of MOSFET switch SW1 is connected to a collector ofbipolar switch SW2. A source of MOSFET switch SW1 and a base of bipolarswitch SW2 are connected to zener diode Z1, resistor R1, and feedbackcontroller 125. An emitter of bipolar switch SW2 is connected to ground.In operation, switch SW1 is turned on and switch SW2 is turned off whenthe LED current is below the desired peak. This mode permits a normaloperation of the front-end components of power supply 120. Conversely,switch SW1 is turned off and switch SW2 is turned on when the LEDcurrent exceeds the desired peak. This limits the peak of the LEDcurrent to a safe level whereby damage to LED light source 110 isprevented. As will be appreciated by one having skill in the art, LEDcontrol switch 127 is particularly useful upon a connection of LED lightsource 110 to an energized power supply 120 whereby capacitor C2discharges stored energy to LED light source 110 with a current having apeak clamped to thereby prevent damage to LED light source 110.

MOSFET switch SW1 can be operated by a conventional gate driver (notshown) or by an illustrated LED PWM dimmer 128.

LED PWM dimmer 128 provides a PWM signal (not shown) to MOSFET switchSW1 in response to an external dim command V_(DC). LED PWM dimmer 128adjusts the duty cycle of the PWM signal to thereby produce a desiredlight output from LED light source 110. LED PWM dimmer 128 isparticularly useful in producing a precise and temperature sensitiveminimum dim level for LED light source 110.

LED PWM dimmer 128 includes a diode D4 and a diode D5 connected to thegate of MOSFET switch SW1. A comparator U3 of LED PWM dimmer 128 is inthe form of an operational amplifier having an output connected to diodeD4 and a non-inverting input for receiving a dimming command V_(DC). Aconventional astable multivibrator circuit 129 of LED PWM dimmer 128 isconnected to diode D5. A ramp generator of LED PWM dimmer 128 includes aresistor R16 connected to diode D5 and a gate of transistor Q2 in theform of a MOSFET. Transistor Q2 can be in other forms, such as, forexample, an IGBT. The ramp generator further includes an operationalamplifier U4. A resistor R15, a resistor R17, a drain of bipolartransistor Q2, a capacitor C7, and an inverting input of comparator U3are connected to a non-inverting input of operational amplifier U4.Resistor R15 is further connected to an output of operational amplifierU4. A resistor R13 is connected to the output and an inverting input ofoperational amplifier U4. A resistor R14 is connected to the invertinginput of operational amplifier U4 and ground. The source of MOSFETtransistor Q2 and capacitor C7 are connected to ground. Resistor R17 isfurther connected to a DC voltage source.

In operation, LED PWM dimmer 128 achieves a precise and temperatureinsensitive minimum dim level for LED light source 110. Specifically,astable multivibrator circuit 129 produces a minimum pulse width (e.g.,T_(ON,MIN) illustrated in FIG. 3). The duration of the minimum pulsewidth is a function of a resistance and capacitance of astablemultivibrator circuit 129. Thus, the minimum pulse width is accurate andtemperature insensitive. The ramp generator produces a ramp signal(e.g., RS illustrated in FIG. 3), which is periodically reset by theminimum pulse width. The ramp signal is supplied to the inverting inputof comparator U3 whereby a comparison of the ramp signal and dim commandV_(DC) yields a target pulse width at the output of comparator U3 (e.g.,T_(ON) illustrated in FIG. 3). The minimum pulse width and the targetpulse width are combined to provide the PWM signal at the gate of MOSFETswitch SW1. As such, the PWM signal consists of the target pulse widthoverlapping the minimum pulse width when the dim command V_(DC) exceedsor is equal to the ramp signal. Conversely, the PWM signal exclusivelyconsists of the minimum pulse width when the ramp signal exceeds thevoltage dim command V_(DC).

In practice, a suitable range for voltage dim command V_(DC) is 0 to 10volts.

Short/Open Circuit Detection

FIG. 4 illustrates one embodiment of short/open detection circuit 130. ALED voltage drop V_(LD) across the LED light source 110 applied betweena node N1 and a node N2, and an input voltage V_(IN) is applied betweennode N2 and a common reference. The LED voltage drop V_(LD) approximateszero (0) volts when LED light source 110 (FIG. 2) is shorted, andapproximates the LED voltage V_(LED) of regulated power P_(REG) (FIG. 1)when LED light source 110 is an open circuit. The input voltage V_(IN)is typically in the range of six (6) volts to sixteen (16) volts. Acomparator U3 in the form of an operational amplifier provides adetection signal V_(DS) at a high level to indicate a “LED outage”condition of LED light source 110 and at a low level to indicate anormal operation of LED light source 110. The “LED outage” condition iseither indicative of a short or open of LED light source 110.

Input voltage V_(IN) in the illustrated embodiment is a dc voltage. Adc-dc type power converter can therefore be used to supply power to LEDlight source 110 (FIG. 2). In alternative embodiments, detection circuit130 can be adapted for use in ac to dc type power converters.

An emitter of a transistor Q3 in the form of a bipolar transistor, and azener diode Z4 are also connected to node N1. Transistor Q3 can be inother conventional forms, such as, for example, an IGBT. A resistor R18,a resistor R21, and a resistor R22 are also connected to node N2. A baseof bipolar transistor Q3 is connected to resistor R18. Zener diode Z4, aresistor R20 and resistor R21 are connected to an inverting input ofcomparator U5. A collector of bipolar transistor Q3, a diode D6, and aresistor R19 are connected to a node N3. Resistor R19 and resistor R20are further connected to the common reference. Diode D6 and resistor R22are connected to a non-inverting input of comparator U5.

For a normal operation of LED light source 110, the LED voltage dropV_(LD) is greater than the base-emitter junction voltage of transistorQ3 whereby transistor Q3 is on, diode D6 is in a non-conductive state,and the voltage at the collector of transistor Q3 exceeds the inputvoltage V_(IN). As a result, the input voltage V_(IN) is applied to theinverting input of comparator U3. The conducting voltage of zener diodeZ4 is chosen to be above the LED voltage drop V_(LD) and therefore zenerdiode Z4 is in a non-conductive state. As a result, a voltage applied tothe non-inverting input of comparator U2 will equate the input voltageV_(IN) reduced by a voltage divider factor established by resistor R20and resistor R21. The output of comparator U5 will be low (e.g., closeto ground) since the voltage applied to the inverting input exceeds thevoltage applied to the non-inverting input.

For an open array condition of LED light source 110, the LED voltagedrop V_(LD) approximates the LED voltage V_(LED) of regulated powerP_(REG), which is chosen to be higher than the voltage of zener diodeZ4. The LED voltage drop V_(LD) is greater than the base-emitterjunction voltage of transistor Q3 whereby transistor Q3 is on and thevoltage at the collector transistor Q3 exceeds the input voltage V_(IN).As a result, the input voltage V_(IN) is applied to the inverting inputof comparator U3. The conducting voltage of zener diode Z4 is lower thanthe LED voltage drop V_(LD) and zener diode Z4 is therefore in aconductive state. As a result, a voltage applied to the non-invertinginput of comparator US will equate a summation of the input voltageV_(IN) and the LED voltage drop V_(LD) minus the conducting voltage ofdiode D6. The output of comparator US will be high (e.g., close to theinput voltage V_(IN)) since the voltage applied to the non-invertinginput exceeds the voltage applied to the inverting input.

For a short array condition of LED light source 110, the LED voltagedrop V_(LD) approximates zero (0) volts. The LED voltage drop V_(LD) istherefore less than the base-emitter junction voltage of transistor Q3whereby transistor Q3 is off, the voltage at the collector transistor ispulled down by resistor R19 and diode D6 is conducting. As a result, avoltage applied to the inverting input of comparator US will equate theinput voltage V_(IN) reduced by a voltage divider factor established byresistor R19 and resistor R22. The conducting voltage of zener diode Z4exceeds the LED voltage drop V_(LD) and zener diode Z4 is therefore in anon-conductive state. The output of comparator US will be high (e.g.,close to the input voltage V_(IN)) since the voltage applied to thenon-inverting input exceeds the voltage applied to the inverting input.

In an alternate embodiment, an additional zener diode or a voltagereference can be inserted in the emitter path of transistor Q3 to detecta voltage level other than less that one base-emitter junction oftransistor Q3.

FIG. 5 illustrates a differential amplification circuit having a voltageoutput V_(O) that can be employed in LED current sensor 25 (FIG. 1) orLED current sensor 26 (FIG. 1). A resistor R23 and a resistor R25 areconnected to an offset voltage source V_(OFF). Resistor R25, a resistorR26, and a resistor R28 are connected to an inverting input of anoperational amplifier U6. A resistor R24 and a resistor R27 areconnected to a non-inverting input of operational amplifier U6. ResistorR23 and resistor R24 are connected. Resistor R28 is further connected toan output of operational amplifier U6.

In operation, the voltages applied to the inputs of the operationalamplifier U6 are lower than the supply voltage V_(dd) irrespective ofthe size of resistor R23. In one embodiment, resistors R25 and R26 arechosen to apply half of the offset voltage V_(OFF) to the invertinginput of operational amplifier U6, and resistors R24 and R27 are chosento obtain a proper common mode rejection (e.g., resistor R28 equaling aparallel combination of resistor R26 and R28). As a result, the gain ofoperational amplifier U6 can be adjusted as desired.

It is important to note that FIGS. 2-5 illustrates specific applicationsand embodiments of the present invention, and is not intended to limitthe scope of the present disclosure or claims to that which is presentedtherein. Upon reading the specification and reviewing the drawingshereof, it will become immediately obvious to those skilled in the artthat myriad other embodiments of the present invention are possible, andthat such embodiments are contemplated and fall within the scope of thepresently claimed invention.

While the embodiments of the invention disclosed herein are presentlyconsidered to be preferred, various changes and modifications can bemade without departing from the spirit and scope of the invention. Thescope of the invention is indicated in the appended claims, and allchanges that come within the meaning and range of equivalents areintended to be embraced therein.

1. A power supply (20) for a LED light source (10), said power supply(20) comprising: a power converter (23) operable to provide a regulatedpower including a LED current and a LED voltage; and a LED controlswitch (24) operable to control a flow of the LED current through theLED light source (10), wherein said LED control switch (24) is furtheroperable to clamp a peak of the LED current during an initial loadingstage of the LED light source (10).
 2. The power supply (20) of claim 1,wherein said LED control switch (24) includes: a switch (SW1) operableto establish a current path from the LED light source (10) to said powerconverter (23) when the LED current is below a peak threshold, saidswitch (SW1) further operable to eradicate the current path when the LEDcurrent is above the peak threshold.
 3. The power supply (20) of claim2, further comprising: a LED PWM dimmer (29) operable to provide a pulsewidth modulation signal to said switch (SW1) in response to an externaldim command, wherein said pulse width modulation signal has a targetpulse width in response to the dim command exceeding a ramp signal, andwherein said pulse width modulation signal has a minimum pulse width inresponse to the ramp signal exceeding the dim command.
 4. The powersupply (20) of claim 3, wherein said LED PWM dimmer (29) includes: anastable multivibrator circuit (129) operable to establish the minimumpulse width in a precise and temperature insensitive manner.
 5. Thepower supply (20) of claim 2, wherein said LED PWM dimmer (29) includes:a comparator (U3) operable to establish the target pulse width inresponse to a reception of the dim command and the ramp signal.
 6. Thepower supply (20) of claim 5, wherein said LED PWM dimmer (29) furtherincludes: a ramp generator operable to provide the ramp signal to saidcomparator (U3) indicative of the minimum pulse width.
 7. The powersupply (20) of claim 6, wherein said LED PWM dimmer (29) furtherincludes: an astable multivibrator circuit (129) operable to establishthe minimum pulse width in a precise and temperature insensitive manner.8. The power supply (20) of claim 1, further comprising: a detectioncircuit (30) operable to provide a detection signal indicative of anoperating condition of the LED light source (10) associated with the LEDvoltage, wherein the detection signal has a first level representativeof a load condition of the LED light source (10), and wherein thedetection signal has a second level representative of either a shortcondition or an open condition of the LED light source (10).
 9. Thepower supply (20) of claim 8, wherein the load operating conditionindicates a magnitude of a LED voltage drop across the LED light source(10) is between zero volts and the LED voltage.
 10. The power supply(20) of claim 8, wherein the short operating condition indicates amagnitude of a LED voltage drop across the LED light source (10)approximates zero volts.
 11. The power supply (20) of claim 8, whereinthe open operating condition indicates a magnitude of a LED voltage dropacross the LED light source (10) approximates the LED voltage.
 12. Thepower supply (20) of claim 1, further comprising: a current sensor (25)operable to sense the LED current flowing through the LED light source(10), said current sensor (25) including an operational amplifier (U6),and means for adjusting a gain of said differential amplifier.
 13. Thepower supply (20) of claim 1, further comprising: a voltage sensor (26)operable to sense the LED voltage applied to the LED light source (10),said voltage sensor (26) including an operational amplifier (U6), andmeans for adjusting a gain of said differential amplifier.
 14. A methodof operating a LED light source (10), said method comprising: providinga regulated power to the LED light source (10), the regulated powerincluding a LED current and a LED voltage; controlling a flow of the LEDcurrent through the LED light source (10); and clamping a peak of theLED current during an initial loading stage of the LED light source(10).
 15. The method of claim 14, further comprising: generating adetection signal indicative of an operating condition of the LED lightsource (10) associated with the LED voltage, wherein the detectionsignal has a first level representative of a normal operating conditionof the LED light source (10), and wherein the detection signal has asecond level representative of either a short operating condition or anopen operating condition of the LED light source (10).